1. Field of the Invention
The present invention relates to a method and related communication device of scheduling request behavior in a wireless communication system, and more particularly, to a method and related communication device of criteria of scheduling request cancellation.
2. Description of the Prior Art
A long-term evolution (LTE) system, initiated by the third generation partnership project (3GPP), is now being regarded as a new radio interface and radio network architecture that provides a high data rate, low latency, packet optimization, and improved system capacity and coverage. In the LTE system, an evolved universal terrestrial radio access network (E-UTRAN) includes a plurality of evolved Node-Bs (eNBs) and communicates with a plurality of mobile stations, also referred as user equipments (UEs). The radio protocol stacks of E-UTRAN is given including a radio resource control layer (RRC), a packet data convergence protocol layer (PDCP), a radio link control layer (RLC), a media access control layer (MAC), and a physical layer (PHY).
In order to utilize shared channel (SCH) resources, a dynamic scheduling (DS) function is used in the MAC. When sharing resources between UEs, MAC in an eNB dynamically assigns downlink shared channel (DL-SCH) or uplink shared channel (UL-SCH) resources by physical downlink control channel (PDCCH) signaling including cell radio network temporary identifier (C-RNTI) to allocate physical layer resources depending on traffic volume, quality of service (QoS) requirements of each UE, and associated radio bearers. Furthermore, in order to optimize data transmission, a semi-persistent scheduling (SPS) is introduced afterward and is also used in the MAC for serving upper layer applications, which generates semi-static size data periodically, e.g. VOIP services, more efficiently than DS. The eNB activates/modifies SPS resources by sending a PDCCH signalling including SPS C-RNTI that is a specific UE identity assigned by the RRC.
According to the current MAC specification, a scheduling request (SR) is used by a UE for requesting UL resources. The SR is triggered when the UE does not have any UL resource allocated for the current transmission time interval (TTI), which implies that a dedicated SR (D-SR, also abbreviated to SR) is transmitted on a resource, e.g. a physical uplink control channel (PUCCH). For DS, if a PUCCH is configured for the UE to send an SR in the current TTI, the MAC instructs PHY to signal the SR on the PUCCH. Otherwise, if no PUCCH for an SR is configured for the UE in any TTI, the MAC performs a random access procedure in which the MAC instructs the PHY to transmit a Random Access Preamble using a selected physical random access channel (PRACH) resource. After the Random Access Preamble is transmitted successfully, the eNB sends a Random Access Response corresponding to the Random Access Preamble to the UE. Besides, a triggered SR is considered pending and is repeated until resources for new transmission are available. Note that the above are mainly illustrates SR behavior in DS. Up to now, there is no further consideration for SR behavior when SPS is introduced.
For DS, the eNB assigns UL resources for a new transmission or a retransmission by PDCCH signaling. How the UE recognizes a new transmission or a retransmission is through a hybrid automatic repeat request (HARQ) process identification (ID) and a new data indicator (NDI). For the same HARQ process ID (implicit for UL grant), the toggled NDI indicates a new transmission or a retransmission. When SPS is introduced, it is specified that the eNB activates/modifies SPS resources by a PDCCH signaling, and the periodic SPS resources after the activation are allocated without PDCCH assignment. SPS modification is that eNB modifies the periodicity of SPS to a bigger or smaller value when UL resource demand for the UE configured with SPS decreases or increases. SPS retransmission is also assigned by PDCCH signaling. Note that the use of NDI in SPS is different from that in DS. One or more specific HARQ process ID(s) is assigned/associated with SPS, and NDI is always set to 0 for activation/modification and 1 for retransmission. The UE can identify the used type of transmission and scheduling through C-RNTI, SPS C-RNTI (or the specific SPS HARQ process ID), and NDI.
Please refer to FIG. 1, which is a comparison table of SR behavior of DS and SPS according to the prior art. For DS, a pending SR is repeated until UL-SCH resources are granted for a new transmission, i.e., a pending SR is cancelled by the allocated UL resource for a new transmission. Compared with the definite SR behavior in DS, SR behavior when SPS is introduced is kind of indistinct. As to SPS retransmission, it is obvious that a pending SR should be repeated since the allocated UL grant is not for new data. Even though NDI concept for SPS is different from that for DS, there is little doubt that the UE has to continue SR after an UL-SCH resource is granted for an SPS retransmission. As to two cases of new SPS transmission, SPS activation/modification and periodic SPS resources, as shown in FIG. 1, it is ambiguous that whether SPS UL resources with PDCCH assignment, e.g. SPS activation/modification, should terminate SR, and whether UL-SCH resources without PDCCH assignment, e.g. periodic SPS UL resources, should terminate SR.
For SPS activation/modification, new UL-SCH resources are assigned by PDCCH and concerns mainly about SPS data. For periodic SPS resources that are not assigned by PDCCH every time, they are also mainly allocated for SPS data transmission. SPS data has a higher priority to be transmitted with UL resources. Up to now, it is not clear whether an SR is triggered by excess SPS data or by non-SPS data when non-SPS data arrives in a transmission buffer in which SPS data already exists. If an SR is triggered by non-SPS data but the triggered SR is cancelled by periodic SPS resources or by an UL resource for SPS activation/modification before any PUCCH resource is available, non-SPS data may sit in the transmission buffer for a long time. Please refer to FIG. 2, which is a timing diagram illustrating a worst case that SPS resources periodically cancel SRs according to the prior art. The cancellation could happen periodically and results in short-lived SRs which never reach the coming PUCCH resources. Consequently, eNB is never informed about the buffered non-SPS data, and the non-SPS data always sits in the transmission buffer.
In addition, the eNB has the right to ignore SR(s) from any UE if it deems proper, and the eNB can also terminate the SR by assigning an UL resource through dynamic scheduling. Please refer to FIG. 3, which is a timing diagram illustrating that an SR is still pending when the transmission buffer is already emptied according to the prior art. As shown in FIG. 3, an SR triggered by non-SPS data is supposed to be not cancelled by SPS resources and is transmitted with an available PUCCH resource, so that the non-SPS data is transmitted latter with an allocated UL grant. And, the next non-SPS data also triggers an SR and is transmitted by an unused SPS resource, which leaves the transmission buffer empty. In this situation, the triggered SR is still pending. In the current specification, it is not clear that what the eNB or the UE should do with respect to a pending SR after the transmission buffer is emptied; however, it is no meaning for the UE continuing the pending SR after the transmission buffer is emptied.